To activate a mechanical transmission emergency operation in an automatic transmission with an automated wet start-up clutch, possibly after a failure of a transmission control device, the flow of power into the transmission is stopped by bringing the start-up clutch into an disengaged condition with the power off. This strategy can result in critical conditions, especially during driving conditions in flowing traffic, because the vehicle without power can only coast.
Engaging the wet start-up clutch is also not possible in such a driving situation for technical safety reasons, since the frictional connection is only maintained by a hydraulic clutch activation pressure. Insofar as the vehicle drive motor stalls with a reduction of the vehicle speed and an engaged clutch, there remains only residual vehicle speed with which important secondary devices driven by the motor, like power brake units or power steering pumps, can no longer be sufficiently powered.
In automatic transmissions with a dry start-up clutch, this clutch is engaged to activate a mechanical dry-running operation so that the output of the vehicle remains connected, via the drive mechanism, with the motor until the vehicle stops. In the process, no critical driving conditions result, but after the vehicle comes to a stop, it cannot be moved anymore or pushed away.
Accordingly, there is need of a control device for a start-up clutch for an automatic vehicle transmission, which initially interrupts the flow of power in the drivetrain, depending on the engine speed and/or output rotational speed of the transmission, when a certain engine speed or transmission output rotational speed is exceeded so that the vehicle won't stall, the secondary devices remain powered and the driver still has the possibility of safely leaving a danger zone, if necessary. In addition, movement of the vehicle is also possible when at a standstill by way of such a control procedure, since the flow of power, between the motor and the transmission, is interrupted.
Against this background, a hydraulic emergency control for a continuously variable transmission is known from DE 199 43 939 A1 in which a clutch allocated to the transmission can be disengaged or engaged, depending on the rotational speed of the vehicle drive motor. As a result, in the event of a disturbance, stalling the drive motor can be avoided upon exceeding of a certain engine speed and starting is made possible upon exceeding a certain rotational speed. Depending on the design of the emergency control, the control signal which depends on the rotational speed can be generated and used as a hydraulic pressure, a pneumatic pressure or an electrical voltage.
In addition, a procedure to control an emergency shifting program for an automatic transmission with a start-up clutch is known from DE 102 38 104 A1 which can be implemented in particular to realize an emergency operation even at standstill of the vehicle, as well as to prevent a reduction of the motor rotational speed below a stall threshold. It is anticipated with this procedure that the emergency shifting program is actuated by a signal dependent on the vehicle speed and/or the motor rotational speed, which can be processed by valve logic and causes frictional connection to the motor to be interrupted in a timely manner during motor braking in order to avoid stalling the vehicle motor.
From DE 103 38 355 A1 a double clutch transmission is also known, which incorporates a first and a second clutch, where for normal operation, a first hydraulic power unit controlled by electronics is available to engage/disengage the first clutch and a second one to engage/disengage the second clutch. Furthermore, a status-maintaining, hydraulic power unit is provided which receives status signals corresponding to the current shift condition of the first and second clutch, via a first and a second hydraulic line, and that are connected via hydraulic control lines with the first and second hydraulic power unit. In the event of a failure of the electronics, the status-maintaining, hydraulic power unit actuates the first and second hydraulic power unit such that, at least in many shift conditions of the two clutches, the shift condition of the clutches, which was present immediately before the failure of the electronics, can be maintained.
Finally, a control valve arrangement to control the start-up clutch of an automatic transmission is known from DE 102 004 020 569.8, which was previously unpublished as of the filing date of this patent application, with which the start-up clutch can be disengaged in a simple, cheap-to-produce and reliable manner during an emergency control situation, when the motor rotational speed and/or power output of the transmission or the driving speed of the vehicle falls below a predetermined value.
The control valve arrangement includes a clutch-regulating valve to control at least a clutch activation device which, during normal operation of the transmission, converts a supply pressure previously furnished into a clutch activation pressure to control a clutch activation device depending on a pilot pressure or an electric pilot signal. The control valve arrangement is also characterized by an activation pressure, dependent on the motor rotational speed and/or transmission output rotational speed, being sent to the clutch-regulating valve or directly to the clutch activation device to achieve an emergency operation of the transmission in the event of the absence of the pilot pressure or the electric pilot signal, where it is held in the engaged position until the named rotational speed rises above the predetermined rotational speed boundary value.
Through this valve arrangement, a cheap to produce and reliably operated control device for emergency driving operation of a vehicle with an automatic transmission is created which is then activated when an electronic transmission control device and/or an electrically operated clutch-regulating valve fails. In this connection, the control pressure, dependent on the rotational speed of the vehicle drive motor and/or the transmission output rotational speed, ensures that the start-up clutch of the automatic transmission, remains engaged for torque transfer by the transmission so long as the vehicle speed and thus the rotational speed of the drive motor does not fall below a stall rotational speed at which the drive motor would cease its combustion engine operation.
Insofar as the vehicle speed is actually reduced during such emergency drive to the extent that stalling of the motor should be expected with an engaged start-up clutch, the known control valve arrangement opens such that the pilot pressure coupled to the control pressure the start-up clutch which, up until then, was transferring torque. A stalling of the motor is thereby advantageously avoided so that important vehicle secondary devices, like a power brake unit or power steering, can continue to operate without problems.
The invention has the basic task of proposing a control valve arrangement of the type named with a different construction with which a hydraulic emergency operation of the automatic transmission is activated and can be maintained when, during the failure of an electronic control device, a predetermined pilot pressure for the valves of the control valve arrangement is absent and a control pressure, dependent on motor or output rotational speed, is sufficiently high.